Conventionally, the upstream end of a turbine engine comprises a fan comprising an impeller formed by a plurality of blades surrounded externally by an annular casing that may be produced from a metal material or a composite material comprising a matrix including reinforcement fibres, such as a polymer matrix, for example epoxy polymer, and reinforcement fibres made from carbon fibres or glass fibres. This casing allows an initial compression of the air entering the turbine engine and also provides a function of confinement of the blades in the event of loss of one of them. The fan casing is surrounded by a plurality of ducts supplying, in particular through a pressurised-air supply duct, at a temperature of around 200° C., equipment of a motor referred to as an APU (auxiliary power unit) used for starting the turbine engine, as well as supplying electricity to the aircraft cabin when the aircraft is on the ground.
In the event of malfunctioning, such as a leakage, of the pressurised-air supply conduit, the air may cause significant local heating of the casing since the temperature of the air is around 200° C. When the casing is produced from metal material, for example aluminium, the heating has no impact on the mechanical integrity of the casing. In the case of a casing with a matrix including reinforcement fibres, its mechanical strength following heating must be able to be guaranteed.
It will therefore be understood that the non-destructive testing of a composite casing with a matrix with reinforcement fibres is particularly important and is all the more so since a composite casing proves to be very expensive.
It has thus been proposed to apply thermosensitive paints to the casing. However, the service life of these paints greatly limits their advantage since an engine may be used for periods greater than the service life of these paints, in particular for aircraft of the long or medium haul type. Furthermore, when an engine is removed, it is subjected in a conventional manner to cleaning by pickling, which causes a total removal of the layer of thermosensitive paint, involving a further step of application of a coat of paint. Finally, though a thermosensitive paint makes it possible to visually note the state of heating of a given region of a casing, it proves to be only an indirect indication of the state of the internal structure of the casing and does not allow precise quantification of the internal structure of the casing.